Slice orientation selection arrangement

ABSTRACT

A slice orientation selection arrangement for magnetic resonance imaging in planes oblique to the normal cartesian coordinates. The coordinates are rotated to applying gradient signals using Euler computations.

FIELD OF THE INVENTION

This invention relates to Magnetic Resonance Imaging (“MRI”) systems andmore particularly to slice selection arrangements for use with such MRIsystems.

BACKGROUND OF THE INVENTION

One of the advantages of MRI systems is a lack of moving parts. Incontrast to CT imaging systems, there are no detectors or radiationsources which translate and/or rotate about the object or patient.Instead of the rotating parts, slices to be images are selected bymanipulating field gradients and pulse sequences to provide images inthe usual sagittal, coronal or transverse (axial) planes. Thus it isknown to vary the static magnetic field with gradient pulses appliedduring the application of RF (Larmor) frequency pulses to select theimaging planes.

For example, consider an MRI system laid out according to the X, Y, andZ cartesian coordinates with the static field applied extending in thedirection of the Z axis. The patient is oriented longtitudinallycoaxially with the Z axis. Generally speaking, sagittal, coronal ortransverse imaging planes are selected by applying the RF pulsesimultaneously with the Y gradient pulse, an X gradient pulse or a Zgradient pulse, respectively.

In contrast to this, in other modalities using equipment such as CTscanners, the detector and/or the X-ray source are rotated. Usuallyrotation and thus data acquisition is accomplished for mechanicalreasons in a transverse plane, about the patient's body. It is true thatthe source of X-ray energy and/or the detectors can be made to swivel soas to image planes at angles to the transverse planes. Nevertheless, theimaging capability is certainly mechanically limited. With MRI systemsthere is no such mechanical limitations; and therefore, theoretically,it is possible to acquire image data from any direction or in any plane.However, instead of mechanical limitations, there are practical,mathematical and processing limitations to obtaining images innon-orthoganal planes. Accordingly, such images in the non-orthoganalplanes have not been used. Those skilled in the art know that when morethan one gradient is simulateously applied during the excitationprocedure, the imaging process will be unduly complicated. The actualgradients and the read or data collect gradient will also have tocomprise multiple gradients. The selection of each of the gradients isfurther complicated by its relationship to the other gradients.

Certain set procedures are used in the prior art to obtain the exacttype of image wanted in MRI systems. For example, the procedure followedin obtaining spin echo images in an orthogonal plane is to apply a planeselection gradient during the application of a shaped, selected RFsaturation pulse signal. The position plane selection gradient isfollowed by a defocusing negative of the plane selecting gradient. Aftera time period “Ta” following the saturation RF pulse, an inversion RFpulse is applied. Again, a plane selecting gradient is applied duringthe echo inducing signal application period. Prior to the application ofthe read gradient, an encoding gradient signal is applied. The fieldgenerated by the encoding gradient signal is mutually orthogonal to eachof the fields generated by the plane selecting gradient and the readgradient. After another time period Ta another inversion pulse isapplied during the application of the plane selecting gradient pulse. Asecond echo pulse is generated at a time period Ta following theapplication of the second inversion pulse. The process is repeated aslong as meaningful “echos” are obtained.

To select a non-orthogonal plane (herein a plane neither parallel nornormal to more than one of the orthogonal XY, XZ and YZ planes) in theobject, at least two orthogonal gradient pulses have to be asimultaneously applied. Consequently, the encoding and reading gradientseach also require at least two simultaneously applied gradient fields ofproper amplitude and width to select the encoding and reading gradients.The complications involved in such a method have deterred imaging in anybut non-orthogonal planes until now. Accordingly, there is a need forthe efficient imaging of non-orthogonal planes with NMR equipment.

It is known in the X-ray medical imaging art to use means for visiblyindicating the location on the patient of the imaging slices. Thus, forexample, in CT imaging, it is known to apply external light lines to thepatient for aiding in aligning the imaging equipment. See for exampleU.S. Pat. No. 4,385,397. Such indicating means have not been applied inthe magnetic resonance imaging art.

BRIEF DESCRIPTION OF THE INVENTION

The slice orientation selection arrangement taught herein is ideallysuited for usage in cooperation with visible orientation indicationsaligned with the patient's organs. The aligning of the visibleorientation indication automatically selects the plane or slice thatwill encompass a portion of the patient or an organ of the patient.

Accordingly it is an object of the present invention to provide meansand methods for imaging in non-orthogonal planes with MRI equipment andfor automatically selecting such planes with reliability and facility.

According to the present invention a method of imaging an object isprovided wherein the slice selection is at any desired angle to theusual XYZ coordinates of an MRI system, the method comprises the stepsof:

subjecting the object to a static strong magnetic field along the Zcoordinate in order to generate magnetic moments extending in the Zdirection,

applying an RF magnetic signal rotating at the Larmor frequency topreturb said magnetic moments and to generate FID signals,

varying said static field during the application of the RF field using afirst and second magnetic gradient for selection an imaging plane notnecessarily orthogonal to either said X, said Y or said Z planes, and

rotating the coordinate system prior to applying the gradient signals sothat location of the magnetic used to select the non-orthogonal imagingplane is automatically accomplished.

A feature of the invention utilizes as the angle of rotation of the XYZcoordinates an angle obtained using visible indicators or the subject todetermine the desired plane of imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features of this invention will be betterunderstood with reference to the following description made inconjunction with the accompanied drawings, wherein:

FIG. 1 is a block diagram of an MRI system using the invention;

FIG. 2 is a showing of planes along the Z axis parallel to the X-Yplane;

FIG. 3 is a planar view of the planes of FIG. 2;

FIG. 4 is a block diagram showing of an embodiment of circuitry used for“rotating” the coordinate axes of an MRI system;

FIG. 5 is a block diagram of an embodiment of control circuitry used inconjunction with the circuitry of FIG. 4;

FIG. 6 is a timing diagram for use with FIGS. 4 and 5;

FIG. 7 is a pictorial and planar showing of one method of automaticallydetermining angles involved in the inventive system; and

FIG. 8 graphically depicts the rotation of the axes to image at anoblique angle.

GENERAL DESCRIPTION

FIG. 1, at 11 shows a typical MRI system. Means, such as magnet 12, areprovided for generating a large static magnetic field. The magnetincludes a tunnel 13, in which a patient is placed during theexamination by the MRI system. Initially, the patient is placed on amobile bed, 14, which is movable into the tunnel. While the patient ison the bed external to the tunnel, a line generating light system showngenerally at 16, can be used to orient and align the system with somestraight line characteristic of the patient, such as the alignment ofthe patients eyes, for example.

The MRI system further includes gradient coils such as X-gradient coils17, Y-gradient coils 18 and Z-gradient coils 19. The usual transmittingand receiving radio frequency (RF) coils are shown at 21. The systemfurther includes a current generator, shown at 22, for generating thecurrent necessary to activate the static magnet. The static magnet canbe resistive type magnet, a superconductive type magnet or a permanentmagnet within the scope of the invention, although in a preferredembodiment, a superconductive type magnet was utilized.

The gradient coils are each activated by appropriate gradientgenerators, such as the X-gradient 23, Y-gradient 24, and Z-gradientgenerator 26. Each of the gradient generators generate the currentpulses necessary to provide the gradient field required in the MRIsystems.

Similarly, the RF coils, used for transmitting and receiving, aresupplied with the appropriate radio frequency shaped pulses provided byradio frequency modulator 27 during the transmitting mode. The radiofrequency modulator is connected to a radio frequency generator 28 and amodulation frequency generator 29, which modulate the carrier and isused for the appropriate shaping of the Rf pulses used in the MRIsystem. The receiver 31 for receiving the free induction decay (FID)signals is connected to the coils 21 through means such as an electronicswitch 30. The electronic switch operates to connect the coils 21 toeither transmit the RF signal from the modulator 27 or to sense the FIDsignals for the receiver equipment 31.

The system is controlled by a processor 32, which directs the system sothat the proper gradients are applied at the proper time and so that theentire system functions in accordance with a specified program. Memorymeans 33 are provided for use with the display 34.

A unique feature of the system 11 is the Euler computation unit 36coupled to the processor. The function of the Euler computation unit isto simplify imaging in planes that are not normal to the regularorthogonal planes. Before the Euler computation unit can be used, it isnecessary to decide on the imaging plane desired.

There are many methods of selecting the imaging plane. For example, theoperator can “write” in the angles that the desired plane makes with theregular orthogonal planes. Such angles define the orientation of aselected plane. The precise location is fixed by proper selection of thegradients and the Larmor frequency.

An exemplary system operates to select an imaging plane by imaging anorgan of the body at least twice in separate parallel planes.Preferably, the two images each include an organ of interest. A thirdplane at an angle to the first two planes and that passes through theorgan is selected. The two preliminary images are used in the selectionof the third plane to obtain angles between the normal orthogonal planesand the selected third plane.

These angles are then used as Euler angles for mathematically shiftingthe regular X, Y and Z orthogonal axes to new orthogonal axes X′, Y′ andZ′ that are used to define the gradients for the newly selected planes.Performing the Euler shift enables transmitting the signals through thesystem in a normal manner. The Euler computation unit automaticallycomputes the X′, Y′ and Z′ axes and automatically determines from thecomputation, according to Euler's method, the proper inputs to the X-,Y- and Z-gradient generators to provide the currents to the X-, Y- andZ-gradient coils to obtain the selected plane.

The system finds use, for example, if a plane through the heart isdesired at an angle that will approximately bisect the heart. It isknown that the heart does not ordinarily lie in a plane parallel to anyof the orthogonal planes in the regular X, Y, and Z coordinates of MRIsystems.

To obtain a non-orthogonal or oblique imaging plane through thepatient's heart, two transverse imaging slices of the patient are takento image the heart along those slices. The slices are shown as plane Aand plane B in FIG. 2. The planes are selected in a manner well known inthe MRI art. For example, using the regular orthogonal system, aZ-gradient pulse is applied after a static field has been establishedwith the patient in the tunnel. During the application of the Z-gradientpulse, a shaped RF saturation pulse at the Larmor frequency is appliedto select the desired XY plane. An encoding signal, for example, AY-gradient encoding signal is applied preferably, but not necessarily atthe same time as X-gradient and Z-gradient focusing pulses are applied.Subsequently, an inversion Rf signal is applied during the applicationof a new Z-gradient signal. During the time period when an echo FIDsignal is obtained from the system, an X-gradient is applied. The echois received and the process is repeated by applying another inversion RFpulse during the application of a Z-gradient signal. The process isrepeated with the application of another Y-encoding gradient. This iscontinued until enough data is obtained to make the image depicted inthe A plane of FIG. 2. The process is then continued by selecting the Bslice plane through the use of a frequency that will cause the resonanceof the hydrogen nuclei in the B plane, due to the applied Z-gradientpulse.

The operator uses the A and B slices to select the new planes. Theoperator displays the A and B slices. On the A-plane image, for example,he overlays a dot or point “a” at some strategic location on the heart,i.e. in the center of the image of the heart. Similarly, in the B plane,he overlays a line 35 at some strategic location, such as through thecenter of a portion of the heart, that is of interest. As shown in FIG.3 the line 35 and the point (a) on the different planes define the newplane that will subsequently be imaged according to the inventiveprocess. The computer processor 32 determines the Eulerian angle phi(0), which is the angle between the overlay line and a line parallel tothe X-axis.

The computer also determines the angle theta (0) which, is the anglebetween the Z axis and a line perpendicular to the B plane overlay lineprojected onto the A plane and passing through the point “a” in the Aplane. In other words, a line drawn from the overlay point “a” in the Aplane normal to the overlay line in the B plane strikes the overlay lineat point M. The angle theta is the angle defined at the intersection ofthe line Ma and a line parallel to the Z-axis. These two angles aresufficient for determining the new orthogonal axes in accordance withEulerian methods. The third angle is not needed because in the methodused the gradient vectors are isolated rather than in the frame ofreference and the Eulerian third shift is not made. However, if thethird shift were made, it would also be found from the angle of theplane to the Y axis. Each new axis is a function of the original axis asdetermined by the following Eulerian equations:

X′=AX,

Y′=AY, and

Z′=AZ,

where A is a product of successive matrices; i.e. A=BCD. (For anexplanation of the Eulerian relationships see the book entitled“Classical Mechanics” by Herbert Goldstein at pages 107 et seq.,published by Addison-Wesley Publishing Company, in 1950.)

The general rotation of the gradient vectors is given by the matrix Awhich is:$\left\lbrack {A = {{B \cdot \left( {C \cdot D} \right)} = \begin{pmatrix}{{{\cos \quad {\theta cos}\quad \varphi} - {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}},} & {{\sin \quad \theta \quad \cos \quad \varphi},} & {{\cos \quad {\theta sin}\quad \psi} + {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} \\{{{{- \sin}\quad {\theta cos}\quad \varphi} - {\cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}},} & {{\cos \quad \theta \quad \cos \quad \varphi},} & {{{- \sin}\quad {\theta sin}\quad \psi} + {\cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{{- \cos}\quad \varphi \quad \sin \quad \varphi},} & {{{- \sin}\quad \varphi},} & {{\cos \quad \varphi}\quad + {\cos \quad \psi}}\end{pmatrix}}} \right\rbrack$

$A = {{B \cdot \left( {C \cdot D} \right)} = \begin{pmatrix}{{\cos \quad {\theta cos}\quad \psi} - {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\sin \quad \theta \quad \cos \quad \varphi} & {{\cos \quad {\theta sin}\quad \psi} + {\sin \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{{- \sin}\quad {\theta cos}\quad \psi} - {\cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\cos \quad \theta \quad \cos \quad \varphi} & {{{- \sin}\quad {\theta sin}\quad \psi} + {\cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{- \sin}\quad \psi \quad \cos \quad \varphi} & {{- \sin}\quad \varphi} & {\cos \quad \varphi \quad \cos \quad \psi}\end{pmatrix}}$

This matrix is obtained from the three matrices which represent therotation around the three major axes X,Y,Z.

The matrix B of the rotation about the Z axis by an angle θ is given by:$B = \begin{pmatrix}{\cos \quad \theta} & {\sin \quad \theta} & 0 \\{{- \sin}\quad \theta} & {\cos \quad \theta} & 0 \\0 & 0 & 1\end{pmatrix}$

The matrix C of the rotation about the Y X axis by the angle φ is:$C = \begin{pmatrix}1 & 0 & 0 \\0 & {\cos \quad \varphi} & {\sin \quad \varphi} \\0 & {{- \sin}\quad \varphi} & {\cos \quad \varphi}\end{pmatrix}$

The matrix D of the rotation about the X Y axis by an angle ψ is:$D = \begin{pmatrix}{\cos \quad \psi} & 0 & {\sin \quad \psi} \\0 & 1 & 0 \\{{- \sin}\quad \psi} & 0 & {\cos \quad \psi}\end{pmatrix}$

For example, the rotation around the Z axis of the vector (X,Y,Z) by anangle θ, is obtained by the following operation: $\begin{pmatrix}X^{\prime} \\Y^{\prime} \\Z^{\prime}\end{pmatrix} = {\begin{pmatrix}{\cos \quad \theta} & {\sin \quad \theta} & 0 \\{{- \sin}\quad \theta} & {\cos \quad \theta} & 0 \\0 & 0 & 1\end{pmatrix}\quad \begin{pmatrix}X \\Y \\Z\end{pmatrix}}$

which results in the following identity:

(X′,Y′Z′)=(X COS θ+Y SIN θ,−X SIN θ+Y COS θ,Z)

In FIG. 8 is shown the rotation of the vector μ by an angle θ around theZ axis to μ′ (i.e. rotation of the projection P of μ on the X-Y plane byan angle θ to p′), rotation of μ′ around the Y axis by an angle φ to u″,(i.e. rotation of the projection Q′ of u′, after its initial rotation byθ around the Z axis, on the X-Z plane by an angle φ to Q″). In thisexample ψ=0.

Note that this system rotates the gradient vectors around the cartesianaxes as distinct from Eulerian rotation in the above referenced bookwhere the frame of reference is rotated.

The rotated new axes are shown in FIG. 8. The new axes are computed bythe Eulerian computation unit 36 as set forth herein in the descriptionof FIGS. 4 and 5 which depict details of unit 36.

The function of the eqipment of the block diagram of FIG. 4 is toperform the operations necessary for rotating the coordinatesautomatically or, on a selective basis, semi-automatically. Registers51, 52 and 53 (shown in FIG. 4) are provided for registering valuesobtained along the usual X, Y and Z coordinates. In a preferredembodiment each register is a 16 bit, three state unit, comprised of two74 LS 373's for example. Fifteen bits are used for inputing the X, Y andZ information.

Means are provided for giving cos and sin functions of the theta and phiangles. More particularly a look-up table 56 provides these functions ofthe theta and phi angles. The theta and phi angles are inputted manuallyor automatically on conductors 57 and 58 which lead into alpha bufferunit 59 and gamma buffer unit 61 respectively. The look-up table 56 isoperated responsive to enable signals or conductor 62 to receive thetheta or phi information on conductor 64. Conductor 64 carries eithertheta information directly from buffer 59 or phi information from buffer61 and conductor 66. Whether theta or phi information is received isdetermined by the signals delivered on conductor 67. If a logic “1”signal is delivered, the phi buffer is operative and if a logic “0”signal is delivered, the theta buffer is operative. An inverter 68provides the enabling signal to the theta buffer 59 responsive to thelogic “0” signal.

Means are provided for projecting the X, Y, Z or X′ values onto otherplanes i.e. multiplying the X, Y, Z or X′ values by the cos and sinvalues. More particularly a digital multiplier 69 is provided coupled tothe X, Y, AND X′ registers 51-54 and to the table look-up unit 56 overconductors 71 and 72 respectively. The output of the digital multiplier69 is stored either in an RA register 72 or an RB register 73 dependingon whether a latch 1 signal or the latch 2 signal is operative to enableregisters R and RP respectively.

Means are also provided for computing the X″, Y″ and Z″ values from theX, Y, AND Z projections. More particularly arithmetic and logic unit(ALU) 74 is provided coupled to the outputs of registers 72 and 73. Theoutput of unit 74 is stored either in X′ register 54 or in the X″, Y″and Z″ registers 76 77 and 78 respectively. The new values of the X″ Y″or Z″ axes appear on conductors 79, 81 and 82 respectively and aretransmitted from unit 36 to processor 32.

Registers 51, 52 and 53 are activated for receiving X, Y, and Z data byindividual strobe signals. For example, a strobe X signal is applied toregister 51 on conductor 86. A strobe Y signal is applied to register 52on conductor 87, and a strobe Z signal is applied to register 53 onconductor 92. In a similar manner data is read into register 54responsive to a latch 3 signal on conductor 93 and data is read out fromregister 54 responsive to X′ select signals on conductor 94. Note thatthe signals to registers 52, 53 and 54 for read-out purposes are logic 0signals.

Registers RA and RB transmit information responsive to latch 1 and latch2 signals on conductors 96 and 97 respectively. The ALU 74 dispatchesits information responsive to an add select signal on conductor 98.Registers 76, 77 and 78 output X″, Y″ and Z″ values responsive to latch6, 4 and 5 signals on conductors 99, 102, and 101 respectively.

The control circuits for providing the control signals discussed withreference with FIG. 4 is shown in FIG. 5 at 111. The control circuit 111comprises a clock circuit 112. One particular embodiment of a clockcircuit is shown, however, many other types of clock circuits could beused within the scope of this invention. In the clock circuit 112, apair of inverter units 113 and 114 are represented. The inverter unitsare operated responsive to the receipt of strobe X, strobe Y and strobeZ signals on conductors 113a, 113b and 114a respectively. Input 114b ofinverter unit 114 is shown as attached permanently to a high. The outputof inverter unit 114 goes through inverter unit 118 and into one inputof one shot bistable unit 119. The other input to the bistable unitcomes from unit 114 through inverter unit 121. Thus if there is anstrobe X, Y or Z signal, then a signal is applied at A1 and A2 whichinitiates the operation of the clock.

The output of the one shot unit 119 appears on a conductor 116 and isdirected to another one shot unit 122 and to one input of flip-flop unit123. The {overscore (Q)} (Q NOT) output of the one shot unit 122 isdirected to the other input of the flip-flop unit 123 and is fed backinto an input of the one host unit 122 through a pair of invertors 124,126 and conductor 127. The Q output which appears on the conductor 128is fed into an invertor a gate 129 that also receives an input fromflip-flop unit 123 over conductor 131.

The output of inverter gate 129 is fed to inverter gate 132. The outputof inverter gate 132 is the clock signal. A clock NOT (CL) signal istaken from the output of circuit 129 for use as required.

The clock signal shown in the timing diagram of FIG. 6 is fed intobinary counter 133 over conductor 134. The counter 33 counts the edgesof the clock pulses to provide four output signals A, B, C, and D. Thus,responsive to the first negative going edge of the clock signal and eachsubsequent negative going edge a cos select signal of a different signis provided at output A of the binary counter 133 and connected toconductor 62. A second signal received from the binary counter 133 isthe ADD select signal at output B connected to conductor 98. The signalchanges sign at every second negative going edge.

All of the outputs of counter 133 are also fed into the one-of tendecoder unit 135. The output of the decoder unit is a high on one of itsten output conductors per output of the binary counter. These signals inconjunction with the clock signals are used to provide latch 1, 2, 3, 4,5 and 6 control signals as well as X, Y, and X′ select signals. Theparticular outputs of the decoder circuit 135 are low only responsive tolows at certain of the outputs of the counter and are high at othercounter outputs. For example, a plurality of AND gates 136, 137, 138 and139 determine the X SEL, Y SEL, Z SEL and X′ SEL signals responsive toparticular outputs of the decoder circuit 135 going low.

The latch 1 signal is obtained responsive to clock signal on conductor141 and signals from the output of decoder 135. The decoder signals aretransferred to four OR gates 142, 143, 144 and 146. The outputs of thesefour OR gates are conducted to the inputs of NAND gate 147. The outputof the NAND gate 147 is the latch 1 signal.

The latch 1 signal is thus obtained for each positive going clocksignal, i.e. when a low is obtained on outputs 1, 4, 5 and 8 of unit135. The latch 1 signal is also inputed to one shot unit 148. The outputof unit 148 is a delayed low directed to OR gates 149 and 151. The otherinput to gate 149 is the 5th output of the one-out-of-ten decoder unit135. The other input to OR gate 151 is the 9th output of decoder 135.

The outputs of OR gates 149 and 151 are the latch 4 and latch 6 signalsrespectively. These latch signals occur responsive to the 5 and 9 lowoutputs of decoder unit 135, respectively, occuring while the delayedlatch 1 signal is high.

The latch 2 signal is obtained responsive to the clock signal onconductor 152 which leads into OR gates 153, 154, 156 and 157. The otherinputs to gates 153, 154, 156 and 157 are the second output, the thirdoutput, the sixth output and the seventh output of decoder 135respectively. The outputs of gates 153, 154, 156 and 157 are conductedto NAND gate 158. The latch 2 signal at the output of gate 158 isconducted through conductor 159 to one-shot delay circuit 161. Theoutput of the delay circuit 161 goes through OR gates 162 and 163. Theother inputs to those gates are the third and the sixth output ofcircuit 135 respectively. The outputs of gates of 162 and 163 are thelatch 3 and 5 signals respectively.

The generation of the control signals is shown more clearly on thetiming diagram of FIG. 6. Responsive to the first clock pulse the binarycounter 133 provides outputs on A. The output on a goes low for everyother clock pulse. The output on B goes low for the 3rd and 4th pulsesthan high for the 5th and 6th pulses, etc. The output on C is high untilthe fifth pulse, then low until the ninth pulse. The “A” signal is usedas the cos/sin sel i.e. cos sel when high and sin sel when low.

The “B” signals is used as the ADD sel control signal. The “C” signal isused as the φ sel signal.

The X sel control signal is generated responsive to there not being ahigh at both the 1 and 4 output of decoder 135. These high outputsgenerate the X sel signal of the timing diagram. During the time periodwhen either 1 or 4 is low an X sel signal is provided as indicated. In asimilar manner the Y sel and Z sel and the X′ sel signals are providedat the output of gates 137, 138 and 139 respectively.

In clock state 1 the following occurs:

The value of the input of the look-up table 56 is selected responsive tothe θ sel signal.

The cos θ value at the output of the look-up table is selectedresponsive to the cos sel signal. The output is a 16 bit word whichprovides an output to the digital multiplier.

The X value from the three state input register 51 responsive to the Xsel signal is provided to multiplier 69.

In the middle of clock state 1 the positive edge of the latch 1 pulse isgenerated.

Thus the X cos θ value is stored in RA register 76.

In the clock state 2, the following occurs:

the sin θ value at the output of the look-up table 56 is selectedresponsive to the sin θ sel signal.

the Y value from the three state input register 52 is selectedresponsive to the Y sel signal.

In the middle of this state a positive edge of the LATCH 2 pulse isgenerated and the value Y sin 0 is stored in register RB.

In the clock state 3 the following occurs:

the value at the input of the look-up table 56 is selected responsive tothe continuing θ sel signal.

the cos θ value of the output of the look-up table is selectedresponsive to the high cos/sin sel signal.

the Y value is obtained from the three state input registers responsiveto the continuing high Y sel signal.

Also in this state the positive edge of the LATCH 3 pulse is generatedwhich causes the X cos θ−Y sin θ value to be stored in the X′ threestate register 54.

In the middle of this state the positive edge of the LATCH 2 pulse isgenerated which stores the value Y cos θ in the RB register.

In the clock state 4 the following occurs:

the φ value at the input of the look-up table 56 is selected responsiveto the continuing φ sel signal.

the sin φ value at the output of the look-up table 56 is selectedresponsive to sin sel signal (cos sel NOT).

the X value from the three state input registers 51 is selectedresponsive to the X sel signal.

In the middle of this state the positive edge of the LATCH 1 pulse isprovided to cause the value X sin φ to be stored in RA register 72.

In clock state 5 the following occurs:

the φ value at the input of the look-up table 56 is selected responsiveto the φ sel signal.

the cos φ value of the output of the look-up table is selectedresponsive to the cos sel signal.

the Z value from the three state input register 53 is selectedresponsive to the Z sel signal.

In this state the positive edge of the LATCH 4 pulse is generated whichcauses the X sin θ+Y cos θ value to be stored in the output “Y”register. In the middle of this state the positive edge of the LATCH 1pulse is again generated which stores the Z cos φ in the R/A register73.

In the clock state 7 the following occurs:

the φ value at the input of the look-up table 56 is selected responsiveto the φ sel signal.

the cos φ value at the output of the look-up table 56 is selectedresponsive to the cos sel signal.

the X′ value from the three state input register 54 is selectedresponsive to the X′ sel signal.

In this state the positive edge of the LATCH 5 pulse is generated whichcauses the Z′ cos ψ−X sin ψ value to be stored in the Z′ output register78. In the middle of this state the positive edge of the LATCH 2 pulseis generated which causes the X′ cos ψ value to be in the RB register73.

In the clock state 8 the following occurs:

the ψ value at the input of the look-up table 56 is selected responsiveto the ψ sel signal.

the sin ψ value at the output of the look-up table 56 is selectedresponsive to the cos sel signal.

the Z′ value from the three state input register 53 is selectedresponsive to the Z sel signal.

Also in the middle of this state the positive edge of the LATCH 1 pulseis again generated which stores the Z′ sin ψ in the RA register 72.

In the clock state 9—the decade sequencer 112 is blocked until a newstrobe pulse arrives. More particularly the high on the 9 output ofdecoder 135 initiates a high signal from one shot circuit 166 to resetone shot circuit 119.

In this state the positive edge of the LATCH 6 pulse is generated whichcauses the Z′ sin ψ+X′ cos ψ value to be stored in the X″ outputregister 76.

The use of the axes X′ and Y′ enables the imaging to be carried out inthe same manner, except that the gradients used are automatically set byEulerian computation unit. For example, instead of a gradient Z, agradient Z′ signal is automatically applied during the application ofthe Rf pulse. That is, GZ′=GZA. Simiaraly, gradient Y′ encoding signaland a gradient X′ sampling signal would be automatically applied duringthe application of the Rf signals in the above explained spin echosystem.

It should be noted that a spin echo system or any other system of MRImaging can be used for obtaining the data desired within the scope ofthis invention. The key thing is that the selection of the planedetermines the new required axes which automatically then enable theproper gradient signals to be applied for the automatic selection of thedesired and newly created plane.

The system illustrated in FIG. 7 for obtaining the new plane uses a slitof light aligned with the patient's eyes for determining the angulardisplacement of the X axes. A light fixture 41 is shown coupled to meanssuch as servo motor 42 for rotating the light fixture. As shown in FIG.7A, the fixture is designed to project a slit of light. The servo motorturns responsive to the rotation of a setting control (not shown) by theclinician. The clinician turns the knob which remotely turns the servomotor and the light fixtures until the slit of light is aligned with thepatient's eyes. The servo control indicates the angular rotationnecessary for alignment. It is assumed herein that the patient's head isaffixed in harness 43 and the angular displacement is thus now 0: thenew coordinates are computed using the angles θ=0, φ=0, and ψ=0.${\begin{pmatrix}{G\quad X^{\prime}} \\{G\quad Y^{\prime}} \\{G\quad Z^{\prime}}\end{pmatrix}\quad A} = {\begin{pmatrix}{{COS}\quad \theta} & {{SIN}\quad \theta} & 0 \\{{- {SIN}}\quad \theta} & {{COS}\quad \theta} & 0 \\0 & 0 & 1\end{pmatrix}\quad \begin{pmatrix}{G\quad X} \\{G\quad Y} \\{G\quad Z}\end{pmatrix}}$

and GX′, GY′ and GZ′=GX cos θ+Gy sin θ, −GX sin θ+GY cos θ and GZ.

The arrangement of FIG. 7 is only one of the many ways of visualalignment visual alignment means can be used for example in the magnetbore with a television camera for maintaining the alignment without aharness. Also, another servo control can be used for aligning the Y′axis.

While the invention has been described using particularly exemplaryaspects of the invention it should be understood that this descriptionis made by way of example only and not as a limitation on the scope ofthe invention.

What is claimed is:
 1. A method of imaging a subject using MRI systemswherein the selected imaging plane is at any desired angle to the usualX, Y and Z coordinate axes of said MRI system, the method comprises thesteps of: (a) subjecting the subject to a strong static magnetic fieldalong the Z axis in order to align magnetic spins in the Z direction insaid subject being imaged; (b) applying magnetic gradient vector pulsesto vary said static field; (c) generating FID signals by applying RFmagnetic pulses rotating at Larmor frequencies to nutate said spins; (d)selecting desired non-orthogonal imaging planes by selectively rotatingthe direction of the magnetic gradient vectors by determined angularamounts about coordinate axes prior to applying the magnetic gradientvector pulses; (e) determining the angular amount to rotate the magneticgradient vectors about the coordinate axis using visual indicators; and(f) projecting said visual indicators onto the image of said subjectbeing imaged.
 2. A method of imaging a subject using MRI systems whereinthe selected imaging plane is at any desired angle to the usual X, Y andZ coordinate axes of said MRI system, the method comprising the stepsof: (a) subjecting the subject to a strong static magnetic field alongthe Z axis in order to align the magnetic spins in the Z direction insaid subject being imaged; (b) applying magnetic gradient vector pulsesto vary said static field; (c) generating FID signals by applying RFmagnetic pulses rotating at Larmor frequencies to nutate said spins; (d)selecting desired non-orthogonal planes by selectively rotating thedirection of the magnetic gradient vectors by determined angular amountsabout the coordinate axes prior to applying the magnetic gradient vectorpulses; (e) imaging in a first plane using the usual coordinate axes;(f) imaging in a second plane using the usual coordinate axes; (g)placing a visible line on the first plane in an area of interest; (h)placing at least a visible dot in the second plane in an area ofinterest; and (i) using said line and said dot to define a new plane,said new plane being the selected non-orthogonal imaging plane, whereinsaid newly defined plane indicates the angular amounts to rotate thedirection of the magnetic gradient vector pulses.
 3. A method of imaginga subject using MRI systems wherein the selected imaging plane is at anydesired angle to the usual X, Y and Z coordinate axes of said MRIsystem, the method comprises the steps of: (a) subjecting the subject toa strong static magnetic field along the Z axis in order to alignmagnetic spins in the Z direction in the said subject being imaged; (b)applying magnetic gradient vector pulses to vary said static field; (c)generating FID signals by applying RF magnetic pulses rotating at aLarmor frequency to nutate said spins; (d) selecting desirednon-orthogonal imaging planes by selectively rotating the direction ofthe magnetic gradient vectors by determined angular amounts about thecoordinate axes prior to applying the magnetic gradient vector pulses;(e) rotating the Z gradient direction through θ degrees to Z′, the Xgradient direction through θ φ degrees to X′, the Y gradient directionthrough ψ degrees to Y′, where θ, φ and ψ are determined angularamounts; and (f) said last named step including multiplying the X, Y andZ gradients by a matrix A defined as follows:$\left\lbrack {A\text{=}\begin{pmatrix}{\cos \quad {\theta cos}\quad \psi \quad {–\sin}\quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi} & {\sin \quad \theta \quad \cos \quad \varphi} & {\cos \quad {\theta sin}\quad \psi \quad \text{+}\quad \sin \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi} \\{{- \sin}\quad {\theta cos}\quad \psi \quad \text{–}\quad \cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi} & {\cos \quad \theta \quad \cos \quad \varphi} & {{- \sin}\quad {\theta sin}\quad \psi \quad \text{+}\quad \cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi} \\{{- \sin}\quad \psi \quad \cos \quad \varphi} & {{- \sin}\quad \varphi} & {\cos \quad \varphi \quad \cos \quad \psi}\end{pmatrix}} \right\rbrack$

$A = {\begin{pmatrix}{\cos \quad {\theta cos}\quad \psi \quad {–\sin}\quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi} & {\sin \quad \theta \quad \cos \quad \varphi} & {\cos \quad {\theta sin}\quad \psi \quad \text{+}\quad \sin \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi} \\{{- \sin}\quad {\theta cos}\quad \psi \quad \text{–}\quad \cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi} & {\cos \quad \theta \quad \cos \quad \varphi} & {{- \sin}\quad {\theta sin}\quad \psi \quad \text{+}\quad \cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi} \\{{- \sin}\quad \psi \quad \cos \quad \varphi} & {{- \sin}\quad \varphi} & {\cos \quad \varphi \quad \cos \quad \psi}\end{pmatrix}.}$


4. A method of imaging a subject using MRI systems wherein the selectedimaging plane is at any desired angle to the usual X, Y and Z coordinateaxes of said MRI system, the method comprises the steps of: (a)subjecting the subject to a strong static magnetic field along the Zaxis in order to align magnetic spins in the Z direction in the saidsubject being imaged; (b) applying magnetic gradient vector pulses tovary said static field; (c) generating FID signals by applying RFmagnetic pulses rotating at Larmor frequencies to nutate said spins; (d)selecting desired non-orthogonal imaging planes by selectively rotatingthe direction of the magnetic gradient vectors by determined angularamounts about the coordinate axes prior to applying the magneticgradient vector pulses; and (e) rotating the Z, X and Y gradientdirection θ degrees to Z, X, and Y respectively, wherein θ is adetermined angular amount for the Z gradient, said method of rotatingincluding multiplying the X, Y and Z gradients by a matrix B, saidmatrix B is defined as: $B = {\begin{pmatrix}{\cos \quad \theta} & {\sin \quad \theta} & 0 \\{{- \sin}\quad \theta} & {\cos \quad \theta} & 0 \\0 & 0 & 1\end{pmatrix}.}$


5. A method of imaging a subject using MRI systems wherein the selectedimaging plane is at any desired angle to the usual X, Y and Z coordinateaxes of said MRI system, the method comprises the steps of: (a)subjecting the subject to a strong static magnetic field along the Zaxis in order to align magnetic spins in the Z direction in said subjectbeing imaged; (b) applying magnetic gradient vector pulses to vary saidstatic field; (c) generating free induction delay (FID) signals byapplying RF magnetic pulses rotating at Larmor frequencies to nutatesaid spins; (d) selecting desired non-orthogonal imaging planes byselectively rotating the direction of the magnetic gradient vectors bydetermined angular amounts about the coordinate axes prior to applyingthe magnetic gradient vector signals; and (e) rotating the Z, X and Ygradients φ degrees to Z′, X′ and Y′ where φ is a determined angularamount for the X gradient by multiplying the Z, X and Y gradients by amatrix C, where C is defined as: $C = {\begin{pmatrix}1 & 0 & 0 \\0 & {\cos \quad \varphi} & {\sin \quad \varphi} \\0 & {{- \sin}\quad \varphi} & {\cos \quad \varphi}\end{pmatrix}.}$


6. A method of imaging a subject using MRI systems wherein the selectedimaging plane is at any desired angle to the usual X, Y and Z coordinateaxes of said MRI system, the method comprises the steps of: (a)subjecting the subject to a strong static magnetic field along the Zaxis in order to align magnetic spins in the Z direction in said subjectbeing imaged; (b) applying magnetic gradient vector pulses to vary saidstatic fields; (c) generating FID signals by applying RF magnetic pulsesrotating at Larmor frequencies to nutate said spins; (d) selectingdesired non-orthogonal imaging planes by selectively rotating thedirection of the magnetic gradient vectors by determined angular amountsabout the coordinate axes prior to applying the magnetic gradient vectorpulses; (e) rotating the Z, X and Y gradient direction by ψ degrees toZ′, X′ and Y′ respectively where ψ is a determined angular amount forthe Y gradient by multiplying the Z X, Y and Z gradients by a matrix D,where D is defined as: $\left\lbrack {D = \begin{pmatrix}{\cos \quad \varphi} & 0 & {\sin \quad \psi} \\0 & 1 & 0 \\{{- \sin}\quad \psi} & 0 & {\cos \quad \psi}\end{pmatrix}} \right\rbrack$

$D = {\begin{pmatrix}{\cos \quad \psi} & 0 & {\sin \quad \psi} \\0 & 1 & 0 \\{{- \sin}\quad \psi} & 0 & {\cos \quad \psi}\end{pmatrix}.}$


7. A system of imaging subjects using MRI systems wherein the selectedimaging plane is at any desired angle to the usual X, Y and Z coordinateaxes of said MRI systems, said system comprising: (a) means forsubjecting the subject to a strong static magnetic field along the Zaxis in order to align magnetic spins in the Z direction in said subjectbeing imaged; (b) means for applying magnetic gradient vector pulses tovary said static field; (c) means for generating FID signals by applyingRF magnetic pulses rotated at Larmor frequencies to nutate said spins;(d) means for selecting desired non-orthogonal imaging planes byrotating the direction of the magnetic gradient vectors by determinedamounts about the coordinate axes prior to applying the magneticgradient vector pulses; and (e) means for determining the angular amountto rotate the magnetic gradient vectors about the coordinate axesincluding means for projecting visual indicators onto images of saidsubject being imaged.
 8. A system of imaging a subject using MRI systemswherein the selected imaging plane is at any desired angle to the usualX, Y and Z coordinate axes of said MRI systems, said system comprising:(a) means for subjecting the subject to a strong static magnetic fieldalong the Z axis in order to align magnetic spins extending in the Zdirection in said subject being imaged; (b) means for applying magneticgradient vector pulses to vary said static fields; (c) means forgenerating free induction decay (FID) signals by applying RF magneticpulses rotating at Larmor frequencies to nutate said spins; (d) meansfor selecting desired non-orthogonal imaging planes by rotating thedirection of the magnetic gradient vectors by determined angular amountsabout the coordinate axis prior to applying gradient vector signals; (e)means for rotating the Z gradient direction through θ degrees to Z′, theX gradient direction through φ degrees to X′, the Y gradient direction ψdegrees to Y′ where θ, φ and ψ are determined angular amounts; (f) saidlast named means including means for multiplying the X, Y and Zgradients by a matrix A defined as follows:$\left\lbrack {A = \begin{pmatrix}{{\cos \quad {\theta \sin}\quad \varphi}\quad - \quad {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\sin \quad \theta \quad \cos \quad \varphi} & {{\cos \quad {\theta sin}\quad \psi}\quad + \quad {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} \\{{{- \sin}\quad {\theta cos}\quad \varphi}\quad - {\cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\cos \quad \theta \quad \cos \quad \varphi} & {{{- \sin}\quad {\theta sin}\quad \psi}\quad + \quad {\cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{- \cos}\quad \varphi \quad \sin \quad \theta} & {\sin \quad \varphi} & {{\cos \quad \varphi}\quad + {\cos \quad \varphi}}\end{pmatrix}} \right\rbrack$

$A = {\begin{pmatrix}{{\cos \quad {\theta cos}\quad \psi}\quad - \quad {\sin \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\sin \quad \theta \quad \cos \quad \varphi} & {{\cos \quad {\theta sin}\quad \psi}\quad + \quad {\sin \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{{- \sin}\quad {\theta cos}\quad \psi}\quad - \quad {\cos \quad \theta \quad \sin \quad \varphi \quad \sin \quad \psi}} & {\cos \quad \theta \quad \cos \quad \varphi} & {{{- \sin}\quad {\theta sin}\quad \psi}\quad + \quad {\cos \quad \theta \quad \sin \quad \varphi \quad \cos \quad \psi}} \\{{- \sin}\quad \psi \quad \cos \quad \varphi} & {{- \sin}\quad \varphi} & {\cos \quad \varphi \quad \cos \quad \psi}\end{pmatrix}.}$


9. The system of claim 7 including: means for imaging in a first planeusing the usual coordinate axes, means for imaging in a second planeusing the usual coordinate system; means for placing at least a line inthe first plane in an area of interest, means for placing at least a dotin the second plane in an area of interest, and means for using saidline and said dot to define a new plane, said new plane being theselected non-orthogonal imaging plane, whereby said newly defined planeindicates the angular amounts to rotate the direction of the gradientvector signals.
 10. The system of claim 7 including means for rotatingthe Z, X and Y gradient directions θ degrees to Z′, X′ and Y′, where θis a determined angular amount for the Z gradient by multiplying the Z,X and Y gradients by the matrix B, where B is defined as:$B = {\begin{pmatrix}{\cos \quad \theta} & {\sin \quad \theta} & 0 \\{{- \sin}\quad \theta} & {\cos \quad \theta} & 0 \\0 & 0 & 1\end{pmatrix}.}$


11. The system of claim 7 including means for rotating the Z, X and Ygradients φ degrees to Z′, X′ and Y′ where φ is a determined angularamount for the X gradient, by multiplying the Z, X and Y gradients bythe matrix C, where C is defined as: $C = {\begin{pmatrix}1 & 0 & 0 \\0 & {\cos \quad \varphi} & {\sin \quad \varphi} \\0 & {{- \sin}\quad \varphi} & {\cos \quad \varphi}\end{pmatrix}.}$


12. The system of claim 7 including means for rotating the Y gradientdirection ψ degrees to Y′, where ψ is a determined angular amount forthe Y gradient by multiplying the X, Y and Z gradients by a matrix D,where D is defined as: $D = {\begin{pmatrix}{\cos \quad \psi} & 0 & {\sin \quad \psi} \\0 & 1 & 0 \\{{- \sin}\quad \psi} & 0 & {\cos \quad \psi}\end{pmatrix}.}$